CTX-M-type extended-spectrum beta-lactamases in Italy: molecular epidemiology of an emerging countrywide problem

Abstract

A nationwide survey of extended-spectrum beta-lactamase (ESBL) production among Enterobacteriaceae, carried out in 2003, showed that CTX-M-type enzymes have achieved a sizeable prevalence among ESBL producers in Italy, mostly in Escherichia coli and, to a lesser extent, in Klebsiella pneumoniae. In this work, we report on the molecular epidemiology of the CTX-M-producing isolates from that survey and on the mechanisms of dissemination of these emerging resistance determinants. The CTX-M-producing isolates were detected in 10 of the 11 participating centers distributed across the Italian national territory, although at remarkably variable rates in different centers (1.2 to 49.5% of the ESBL producers). All CTX-M determinants were of group 1, with CTX-M-15 and CTX-M-1 being the most prevalent variants (60% and 35%, respectively) and CTX-M-32 carried by a minority (5%) of isolates. Each variant was detected both in E. coli and in K. pneumoniae. Genotyping of the CTX-M-producing isolates by random amplification of polymorphic DNA revealed a notable diversity, especially among those producing CTX-M-1, while clonal expansion was evident with some CTX-M-15-producing strains. Mating experiments revealed a higher overall transferability of bla(CTX-M-1) and bla(CTX-M-32) than of bla(CTX-M-15). Coresistance to quinolones and aminoglycosides was overall higher with the CTX-M-15-producing isolates. The present results indicate that CTX-M-producing strains are now widespread across the Italian territory and underscore the emerging role of these ESBL determinants in the European setting. They also reveal notable differences in the dissemination mechanisms of genes encoding different CTX-M variants of the same lineage.

FIG. 1.

Map of the Italian territory showing the locations of the centers participating in the study, the prevalence of CTX-M producers observed among the ESBL-positive isolates from each center, and the CTX-M variants detected in each center.

FIG. 2.

Dendrograms based on RAPD typing showing genetic relatedness among CTX-M-producing E. coli isolates (A) and among CTX-M-producing K. pneumoniae isolates (B) from various centers. The vertical dotted line indicates the 0.90 similarity score adopted to assign isolates to the same lineage. The origins of isolates are also indicated (abbreviations are the same as in Fig. 1). Lineages for which conjugative transfer of the CTX-M determinant was demonstrated are underlined.